In the treatment of tumours and other disorders by radiotherapy, a beam of high energy radiation is directed towards the target area and causes tissue damage to the tissue therein to weaken the tumour, and eventually, destroy it. In order to prevent collateral damage to healthy tissue surrounding the tumour, the beam is typically shaped so as to reduce areas of overlap and is directed towards the target area from a number of different directions, all centred on the target area. In the past, this was often by way of ceasing the treatment, repositioning the source and/or the patient, and then continuing. Modern radiotherapeutic apparatus however allows for the source to be continuously rotated around the patient and to be shaped by so-called multi-leaf collimators to a desired external shape, which may change with time. The rotation angles, speeds and beam shapes are calculated in advance so as to deliver a three-dimensional dose distribution to the patient which is maximised in the area of the tumour and minimised outside the tumour, particularly so in sensitive areas of the patient.
This type of treatment requires careful control of the position of the patient so that it can be correlated correctly with the dose distribution that will be delivered. Accordingly, such radiotherapeutic apparatus is commonly provided in combination with a diagnostic x-ray source and an associated detector. This diagnostic source may be integrated in the therapeutic head, or it may be spaced around the rotating gantry by (for example) 90 degrees. This allows an investigative rotation to be made with the diagnostic source in order to verify that the patient is in a correct position.
Constraints such as the permissible speed of rotation of the therapeutic source mean that, which a patient in place and awaiting treatment, there is typically only time for only one or two diagnostic rotations. Accordingly, techniques such as cone beam CT reconstruction are used to prepare a three-dimensional volume image of the patient in position prior to treatment. These use a three-dimensional (cone) shaped beam which emanates from the diagnostic source and provides a series of two-dimensional images of the patient from different directions as it is rotated. These are combined using known algorithms so as to produce a three-dimensional volume image.
Such an image is typically not suitable for the treatment planning stage. The volume images for this purpose are prepared on an investigative CT apparatus which uses a two-dimensional fan beam detected by a one-dimensional detector. This fan beam is rotated very quickly around the patient, and the apparatus and/or the patient is indexed longitudinally so that a series of parallel slices are obtained. Each slice is a two-dimensional section through the patient derived from the plurality of one-dimensional images. A three-dimensional volume image of the patient is derived by stacking the adjacent slices. Such an image is generally of a very high quality since scattering artefacts are minimised by the use of a one-dimensional detector and a two-dimensional beam. Further, the individual pixel values are carefully calibrated to a scale known as the Hounsfield scale. According to this scale, an electron density corresponding to water is given the value zero, an electron density corresponding to air is given the value of minus 1000, and all other electron densities are mapped to a straight line relationship governed by these two points.
As a result of scattering within the three dimensional cone beam (and other difficulties) a cone beam CT image includes artefacts such as the well-known “cup” artefact, in which the apparent densities towards the middle of the image are depressed. As a result, the individual pixel values are not generally calibrated to the Hounsfield scale. Nevertheless, such cone beam CT images are of sufficient quality to discern structures within the patient which can be compared with previously obtained investigative images either to confirm that the patient is in the correct position or to derive a vector indicating the positional error in the relevant six degrees of freedom (three translational directions and three rotational directions). This image can then be passed back to the apparatus and used, for example, to adjust the patient position via a motorised patient couch. The treatment can then be delivered with confidence as to its positional accuracy.
If the diagnostic images obtained during treatment were of a suitable quality, they could be used for treatment planning. This would allow adaptive treatment planning to be realised, in other words to investigate the state of the patient at the time of treatment and observe changes within the patient such as movement of the tumour, growth or reduction of the tumour, or the like. In tumours set within relatively mobile areas of the patient such as the abdomen or the lungs, it is possible for the tumour to move and adopt a different position relative to sensitive structures of the patient. This could justify re-running the treatment planning programmes, and the ability to do so would allow the dose distribution to be adapted to the more recent state of the patient. At present, a diagnostic image taken immediately before treatment could be used to revalidate or correct the treatment plan overnight, prior to a subsequent treatment, but with the development of improved computing power over time, this could potentially be done immediately before each and every treatment.
Recent work has therefore been devoted to improving the quality of cone beam CT imaging so that the pixel values can be reliably calibrated in Hounsfield units (and therefore made acceptable to standard treatment planning systems), and in the elimination of artefacts through the improvement of the detectors and the reduction, where possible, of scattering within the cone beam.